Aux Battery

Hello, so basically after a 26 hour
drive on my Sprinter I came to a stop
and went to sleep without turning off
my lights...I know rookie mistake but
that’s not the problem, Fortunately I
had a Truck stop auto shop right next
to me when this happened so I went
there to get help on jump starting my
van, the mechanic told me he will be
able to jump start it in no time and
while I was in the back of the van
moving some stuff I did not notice
how he connected the jumper cables
to the Aux Battery instead of the
starting battery and tried to jump it a
few times before I intervened and told
him that it was a aux battery and not
the main one to which I got a reply
back saying “it does not matter, it
should jump start either way” of
course I had my doubts and thought
it was completely stupid but didn’t
say anything else since the guy said
he had worked on sprinters for a
while now, after a few more failed
attempts he opened the starter
battery and jumped it (worked on the
first try) and now my check engine
light came on, the van works fine but
my question is why did the light come
on? Did he fry some things in the
process?